The present invention relates to a rectangular and sealed battery, and more particularly to a structure for sealing an outer can and a cover.
In recent years, there has been a significant improvement in performance and reduction in size of electronic apparatuses, represented by video cameras and headphone stereo units. Therefore, improvement in heavy load characteristics and high energy densities of secondary batteries, serving as power sources of the electronic apparatuses, have been required.
As the secondary battery for use in the foregoing electronic apparatuses, lead secondary batteries and nickel-cadmium secondary batteries have been known. Recently lithium-ion secondary batteries having higher performance characteristics have been used. As for the shape of the secondary battery, a rectangular shape has widely been used as compared with a cylindrical shape because the space can effectively be used after the rectangular secondary battery has been mounted on the apparatus.
A conventional rectangular battery of the foregoing type, as shown in FIG. 1 has a structure that a power generation element (a laminate of a positive electrode and a negative electrode formed through a separator) is accommodated in an outer can 81. Moreover, an opening of the outer can 81 is closed by a flat cover 82 so that a sealed structure is formed.
The outer can 81 is a steel can having substantially the same thickness throughout the overall body from an opening 81a to a body 81b. Usually the outer can 81 is plated with Cu/Ni having a thickness of several xcexcm.
The cover 82 is manufactured by stamping a steel plate, and by performing machining, such as drawing or casting, to have a size substantially the same as or slightly smaller than that of the opening of the outer can 81. Also a terminal hole 84, through which a counter terminal 83 is inserted, is formed in the cover 82. The terminal 83 is, through a gasket 85, inserted into a terminal hole 84 of the cover 82, and then the leading end of the terminal 83 is crimped. Thus, the terminal 83 is integrally secured to the cover 82. As a result, either of electrodes of the battery is formed. When the foregoing electrode is electrically connected to a positive electrode of the power generation elements accommodated in the outer can 81, the foregoing electrode serves as a positive-electrode terminal. The structure for joining the terminal 83 is not limited to the above-mentioned structure. Another structure may be employed if the structure is able to maintain electrical insulation with respect to the cover 82 and hermeticity of the inside portion of the battery.
An outer end 82a of the cover 82 is joined to the inner surface of the outer can 81, and then laser welding or seam welding is performed from an upper position. Thus, the cover 82 is sealed.
Weight reduction of a rectangular and sealed battery of the foregoing type has been required. In a case of a rectangular and sealed battery having a size 6 mm (t)xc3x9730 mm (w)xc3x9748 mm (h) and overall weight of about 25 g (including about 11 g which is the weight of the can), reduction in the overall thickness (0.4 mm) of the outer can (to, for example, 0.2 mm) enables the overall weight to be reduced to about 20 g.
When an amount of melting of the outer can 81 occurring during a seam welding operation is considered to maintain the reliability of the seam welding operation, the thickness of the outer can 81 cannot be reduced to be smaller than 0.3 mm. Therefore, there arises a problem in that the weight cannot satisfactorily be reduced.
Therefore, an attempt has been made to change the material of the outer can 81 and the cover 82 from steel to an aluminum material having a low specific gravity. Since the outer can 81 and the cover 82 must have required strength, the thickness of the outer can must be 0.5 mm or greater. Therefore, another problem arises in that the capacity of the battery is reduced.
Since the cover 82 of the rectangular and sealed battery structured shown in FIG. 1 is formed into a plate-like shape, satisfactory flexural stiffness cannot be realized. Thus, there arises a problem in that deformation easily occurs even in usual handling. If the cover 82, which has been deformed, is joined to the outer can 81, a stepped portion is formed in the seam welded portion. Thus a defect (a pin hole or a crack) is easily caused in welding. Moreover, position control of an end 81a of the opening of the outer can 81 is not performed in the vertical direction (in a direction into which the cover 82 is inserted). The foregoing fact causes a stepped portion to be formed in the seam-welded portion and a defect (a pin hole or a crack) to occur in the welded portion. If a pinhole is formed in the welded portion, leakage of electrolyte and introduction of water into the inside portion of the battery occur. In this case, the commercial value of the sealed battery deteriorates excessively. If a crack occurs, the strength of the welded portion excessively decreases. If the internal pressure is raised in a case of overcharge or during preservation at high temperatures, the crack spreads out from the welded portion. In the foregoing case, there is apprehension that the contents of the battery leak and apparatus is contaminated.
To prevent the defect occurring in welding, the gap between the cover 82 and the outer can 81 which are joined to each other is required to be minimized. In this case, an accuracy of the dimensions of the shape of the cover 82 and an accuracy of the dimensions of the inner surface of the outer can 81 must be improved. Thus, there arises a problem in that the costs of the elements cannot be reduced. If the gap between the joined elements is narrowed, that is, reduced, the cover 82 cannot easily be inserted into the outer can 81. In this case, there arises a problem in that the productivity in an automatic assembling process deteriorates.
To solve the above-mentioned problems, a first object of the present invention is to reduce the weight of a rectangular and sealed battery such that the reliability of seam welding is maintained and reduction in the capacity of the battery is prevented.
Moreover, a second object of the present invention is to prevent defects occurring in a welding process such that deterioration in the productivity in a process for assembling the rectangular and sealed battery is prevented.
The inventor of the present invention has found that the first object can be achieved by reducing the thickness of the body of the outer can of a rectangular and sealed battery and by providing a flange or a tapered portion for an end or rim portion of the opening of the outer can. Moreover, the second object can be achieved by providing a stepped portion for the outer end of the cover to form a flange. Thus, the present invention has been achieved.
That is, a rectangular and sealed battery which is capable of achieving the first object comprises: an outer can for accommodating a power generation element; and a cover for closing an opening of the outer can such that the outer can and the cover are welded to each other, wherein the thickness of a body portion of the outer can is smaller than the thickness of the rim portion of the opening.
A rectangular and sealed battery which is capable of achieving the second object has a structure according to that of the foregoing rectangular and sealed battery, wherein a flange in the form of a stepped portion is formed around the outer end of the cover, an end of the opening of the outer can is brought into contact with the outer surface of a projection of the cover formed by the stepped portion so as to be joined to the outer surface, and the end of the opening is butt-welded to the flange.
The first rectangular and sealed battery noted above according to the present invention has the structure that the thickness of the body of the outer can is reduced while the thickness of the end of the opening or rim portion of the outer can, which is seam-welded to the cover, has the same thickness as that of the conventional battery. Therefore, the weight of the rectangular and sealed battery can be reduced while the reliability of the seam-welded portion is maintained. The inner diameter of the body of the outer can is enlarged while the outer diameter is not changed. Therefore, the capacity required for the battery can be enlarged, causing an active material for the electrode to be enclosed in a quantity larger than that permitted for the conventional battery. As a result, the capacity of the battery can be enlarged.
The second rectangular and sealed battery according to the present invention has the structure that the flange having the stepped shape is formed around the outer end of the cover, and the end of the opening of the outer can is joined to the projection of the cover formed by the stepped portion. Since the end of the opening of the outer can is abutted against the flange, the position of the outer can is limited. Moreover, the outer can is brought into contact with the outer surface of the projection. Therefore, the second aspect of the rectangular and sealed battery according to the present invention enables the outer can and the cover to stably and easily be joined to each other. Therefore, a reliable seam welding operation can be performed such that formation of a pinhole and a small crack can furthermore be prevented.